The invention relates to a process and an apparatus for applying a coating, preferably a bottom coating, to at least one side, optionally to both sides, of a leather, namely to the grain side and/or to the flesh side of a grain leather opposite this grain side, or to one or both sides of a split leather, and a leather produced by such a process and provided with a coating, preferably a bottom coating.
It is already known that the visible side of a leather can be provided with a dressing. An aqueous plastic dispersion is applied to the leather thereby and is allowed to solidify by supplying heat. It is known that the prepared hollow microspheres can be mixed with this aqueous plastic dispersion for the formation of a foam structure. It has also already been proposed to add to the aqueous plastic dispersion compact particles which contain a blowing agent and from which hollow microspheres are subsequently formed by application of pressure and heat in the solidified plastic dispersion.
It has also already been proposed to provide the back opposite the visible side of the leather with a foam coating which consists of a solidified plastic dispersion containing hollow microspheres. Thus, AT 005281 U1 discloses a leather in which the thick foam coating provided on the back is formed from an aqueous plastic dispersion which, on application, contains both prepared hollow microspheres and compact particles from which hollow microspheres are formed by supplying heat. This thick foam coating has a cushioning function; it has more than 10% of open cells and cannot be used as the bottom coating.
AT 006040 U1 has disclosed a leather and a process for the production hereof, in which the back of the leather opposite the visible side is provided with a foam finish which is formed by solidification of a plastic material applied in liquid form and containing prepared hollow microspheres by the brief action of hot air. The hot air has a temperature of more than 280° C., preferably a temperature between 350° C. and 650° C. and is therefore very hot and dry, which leads to the natural moisture being extracted from the leather and to the latter shrinking and hardening. Furthermore, owing to the considerable action of heat, the hollow microspheres in the uppermost region of the foam finish are frequently destroyed. This coating cannot be used as the bottom coating for building up further dressings.
DE 20318311 U1 discloses a full-grain leather in which defects in the grain layer are corrected by a plastic filling material which consists of a solidified plastic dispersion containing hollow microspheres formed from compact particles by supplying heat. The formation of the hollow microspheres is effected by application of pressure and heat at a temperature of at least 100° C. This known formation, too, has the disadvantage that the leather is adversely affected at the required pressure and the temperature used. In particular, however, the defects formed by indentations are completely filled by the plastic filling material in this formation since the leather which is resilient under pressure recovers after a pressure relief, whereas the plastic filling material experiences no increase in volume, and hence the plastic filling material remains recessed at its top relative to the leather surface. In order to achieve a completely level surface, it is therefore necessary to buff the grain side of the leather to such a depth that the entire surface is level, with the result that the quality of the leather is adversely affected and it is considerably reduced in value. Moreover, the temperature increase required for the expansion of the compact particles for the formation of the hollow microspheres does not take place in the lower regions of the indentations in the grain layer on economically acceptable application of pressure and heat, so that this lower region has no hollow microspheres or only a small number thereof. With the use of higher temperatures over a longer period, hollow microspheres do form in this region but the leather is compacted even more, which leads to further hardening. Moreover, the loose-grained character is fully restored after subsequent drumming of the leather hide, especially in the belly region of a cattle hide.
Repairing of grain damage is also attempted by the so-called stucco method. In this process, a plastic dispersion which can be applied with a spatula and often also contains prepared hollow microspheres is applied manually or mechanically to the leather. On drying, the material shrinks by its proportion of water. The filled areas are thus deeper and for this reason too the grain side has to be subsequently buffed.
Both those processes in which the hollow microspheres are formed from compact particles by means of heat and pressure in the solidified plastic dispersion and those processes in which the hollow microspheres are formed by means of infrared radiation or very hot air have disadvantages and are not suitable for the production of a bottom coating. If hot air is used for supplying heat for the formation of the hollow microspheres, hollow microspheres rapidly form at the surface of the solidified plastic dispersion; they form there a heat-insulating layer and prevent the heat from penetrating to a sufficient degree into the deeper zones in order to form hollow microspheres from the compact particles there too. If, in order to avoid this disadvantage, the temperature of the hot air is increased, the already formed hollow microspheres on the surface of the solidified plastic dispersion are destroyed thereby and the leather hardens and shrinks. Similar behaviour is encountered with the use of infrared radiation.
It was therefore the object of the present invention to avoid the disadvantages of known leathers provided with a foam coating and to provide a leather which has a foam coating applied to at least one side, optionally to both sides, which can be produced in an economical manner, in which shrinkage and hardening during the formation of the foam coating are avoided and in which defects at the surface of the leather can be eliminated without an existing grain structure being damaged by subsequent processing of the top. Furthermore, it is the object of the invention to provide a leather which eliminates or at least considerably reduces the loose-grained character which is always present in the case of the soft full-grain cattle leather, especially in the belly region, and which is provided with a bottom coating which is also firmly bonded to the full grain side of the leather and has a surface on which finish layers can be applied without problems by any desired method, these finish layers forming a close and firm bond to the bottom coating.